The present invention is directed to microwave couplers or transition devices. In particular, it is directed to a transition between a waveguide and a stripline.
Two of the commonly used propagation media for microwaves are waveguides and striplines. A waveguide is often a hollow rectangular conduit in which microwaves are intended to propagate in the TE.sub.10 mode. In this mode of propagation, the direction of the electric-field vector is the direction perpendicular to the broad walls of the waveguide and normal to the direction of propagation, while the magnetic-field vectors have components in the direction of propagation.
A stripline typically is made of two parallel ground-plane conductors between which dielectric layers are disposed. Between the dielectric layers is a relatively narrow conductor disposed midway between the planes of the ground-plane conductors and extending in the direction of propagation. The electric-field lines extend from the center conductor to the ground-plane conductors and vice versa. The ground-plane conductors extend much farther in the direction transverse to the path of propagation than does the center conductor, and the electric-field distribution at the surface of the center conductor approximates the field distribution in a coaxial cable. Accordingly, microwaves traveling through a stripline propagate in a TEM mode, in which both the electric field and the magnetic field are perpendicular to the direction of propagation.
It is sometimes desirable to employ waveguides as the propagation media in some parts of a microwave system and to employ striplines in others. It is therefore necessary to provide transitions between the two media.
In the past, such transitions have typically employed probes that extend from the stripline partway into the waveguide through a rectangular slot either in one of the broad walls of the waveguide or in a shorting plate at the end of the waveguide. In both cases, the probe is provided by cutting away a portion of the stripline to leave a narrow portion that can extend through the slot. At the end of this narrowed portion, which extends into the interior of the waveguide, the ground planes are removed to leave only the center conductor as the probe.
Such transitions yield good matching characteristics through a reasonable range of frequencies. However, they limit the power-handling capability of the system because the probes tend to arc at higher power levels.
Additionally, the transition can be a weak mechanical link in the system. The narrowed stripline section that extends into the slot in the waveguide is weaker mechanically than the waveguide or the larger sections of stripline, and this makes the transition subject to damage when it is not handled carefully.
It is accordingly an object of the present invention to provide a transition between waveguide and stripline propagation that imposes less of a power limitation than previous transitions do. It is a further object to provide a transition that is stronger mechanically.